Prior art radiant energy collectors can include fluid carrying channel devices which are coated with, for example, a black, radiant energy absorbent material. As the channel members conduct fluid therethrough, the radiant energy, as from the sun, absorbed by the black coating is re-radiated, conducted and convected to the moving fluid. In such an arrangement, the black absorbent covering is generally much hotter than the fluid being heated. As a consequence, the thermal efficiency of such a system is not as great as would be desirable. Further, the channel member is subject to thermal stress and subsequent breakdown due to the high temperatures developed and also due to cyclical heat-ups and cool-downs as, for example, happens during the non-operational and night shutdown periods for such a solar system. Further, there is always the possibility that hot spots can develop on the channel member resulting in the burning through of the channel member.
In actuality, a device for collecting solar radiation using the above principle generally requires a plurality of channel members, through which fluid must be pumped, in order to provide enough surface area which is coated with the black absorbent material to adequately and thoroughly heat the fluid. Thus, an additional disadvantage of this system is that there are significant pumping losses from pumping the fluid through the various bends and other inherent restrictions. Also, such devices are generally quite heavy, and there is always the possibility, as indicated above, that the hardware which connects the conduits will expand and contract at a rate different from the conduits themselves and thus eventually lead to leakage problems and a malfunction of the system.
In another prior device that is disclosed in U.S. Pat. No. 4,055,948, issued on Nov. 1, 1977 to Robert A. Kraus, a solar thermal radiation absorption and conversion system includes multiple arrays of sun-tracking mirror heliostats. The heliostats focus incoming thermal radiant energy through a highly light-transparent, radiation receiving, fluid containment shell located on top of a tall central tower. A radiant, heat asbsorbing fluid passes through the containment shell and is heated. The fluid includes minute particles of colloidal size suspended in a transparent heat, transfer fluid. The fluid is channeled between a first highly light-transparent and a second highly light reflecting wall of the shell. The minute particles, which are defined as being dull-black and non-reflecting, absorb the radiant energy flux and readily heat the fluid stream in which they are suspended. The thus heated fluid is sent through a heat exchanger where heat is transferred to a secondary fluid for driving a turbine. The cooled fluid, including the particles, is returned to the containment shell for reheating. In this system, the primary fluid which includes the particles must be isolated by the secondary fluid so that the particles do not come in contact with the fast turning turbine blades and cause damage thereto. Further, in such a system, there is always the problem of uneven heating or the development of hot spots which tend to shorten the life of the system by burning through the apparatus. Another problem associated with this device is that the particles agglomerate, reducing their light collection and thermal efficiency and also can collect on or about the first highly light-transparent wall, reducing the radiation which reaches the moving fluid with the colloidal suspension of particles.